Various imaging techniques, such as X-rays, fluoroscopy, ultrasound, computed tomography (CT), and magnetic resonance imaging (MRI) play an integral role in a wide variety of medical procedures. The term “image assisted” may be used to describe medical procedures utilizing some type of imaging technique to guide the medical procedure.
The incorporation of image guidance systems into various procedures allows a physician to correlate a desired location of a patient's anatomy to images taken pre-operatively or intra-operatively using various imaging modalities such as x-rays, ultrasounds, CT scans, MRI's, etc. The use of image guidance systems also imparts the ability to look through superficial layers of anatomy to visualize deeper targets of interest. Further, image guidance systems provide the guidance needed to access target areas of interest within the patient's anatomy through the use of pre-defined entry points and/or target zones. Often, physicians rely heavily on imaging systems when a target cannot be directly visualized in order to avoid damage to surrounding anatomical structures and to minimize unnecessary tissue trauma.
There are at least two “spaces” that may be used in image guidance systems. The first may be referred to as the “image space,” which may represent the imaging acquired prior to or during a procedure, such as an MRI scan of a specific anatomical area performed before surgery. From cross-sectional imaging, a three-dimensional data set may be constructed using the first image space's coordinate system, usually expressed as a Cartesian system with an arbitrary origin and principle axis. The second space may be the actual physical space surrounding the patient. This is often restricted to a specific anatomical part, such as the head, lower back, hip joint, etc., in order to improve local resolution and system performance. An image guidance system may include a mechanism for accurately measuring position within the patient's physical space, much like a tracking device. The tracking device may have its own coordinate system which may be different from that of the “image space.” In order to provide flexibility, a “reference” may be held in a rigid relationship relative to the patient's anatomical area of interest. The reference can serve as an arbitrary origin of the patient's physical space and all three-dimensional spatial measurements of the patient's physical space can be expressed relative to the reference. The use of a reference can allow for the movement of the image guidance system and/or the movement of the target anatomical region of the patient without losing registration or affecting guidance accuracy. Thus, the tracking device or reference may be used for spatial recognition to read the coordinates of any point in three-dimensional space and allow accurate tracking of the physical space around the patient. An image guidance system also may include various probes to allow tracking of instruments (e.g., surgical instruments, endoscopic tools, biopsy needles, etc.) during operation to provide flexibility with regards to navigational options. The probe may also act as the tracking device or reference.
After the two coordinate systems have been established, the image space may be correlated to the physical space through a process known as registration. Registration refers to the coordinate transformation of one space into another. This is usually a linear and rigid transformation in which only translation and rotation takes place and scaling or local deformation transformations are not necessary.
Once registration is completed, a probe or other device may be used to touch various anatomical structures on the subject (physical space), and the corresponding images of the same anatomical structures may be displayed (image space). The image guidance system may also include multi-planar reconstruction capabilities that can display three-dimensional image datasets in any arbitrary plane allowing users to view surrounding structures in any arbitrary direction.
An image guidance system may include an information processing unit (e.g., a computer). The information processing unit can load a patient's pre-operative and/or intra-operative images and run software that performs registration of a patient's image space to the patient's physical space and provide navigational information to the operator (e.g., surgeon). The software may also include the ability to perform multi-planar reconstructions and targeting/trajectory planning to identify specific entry points, trajectories, target zones, etc. More advanced functions may include image fusion capabilities across imaging modalities such as fusing CT imaging data with MRI imaging data, as well as advanced image segmentation to provide surgeons with live intraoperative guidance. For example, advanced image segmentation may include extracting image information of a patients inner anatomy, (e.g., a tumor, blood vessels, tissues, etc.), rendering three-dimensional models of these structures, and then visually overlaying these structures on a display screen in a manner that shows the relative depth of the tissues/structures inside the patient (e.g., the depth of the tissues/structures relative to the patient's surface anatomy, skin, other tissues/structures, etc.). In this manner, a virtual three-dimensional view of the patient's inner and outer anatomy may be presented to the operator to help the operator visualize the inner locations and depth of tissues/structures inside the patient relative to the patient's surface anatomy.
There are many different ways of implementing an image guidance system. For example, an optical system may include a stereo camera (i.e., two cameras mounted a known fixed distance apart) that cooperate to provide accurate three-dimensional localization. The method of tracking in this example can be passive or active. In passive tracking, the system can emit infrared radiation (usually through a ring of infrared light emitting diodes, or LED's, mounted around each camera) and passive optical markers can reflect the radiation back to the cameras to allow the markers to be seen by the cameras. The markers can be small spheres of a pre-defined diameter coated in a reflective coating that may be optimized for the wavelength of infrared radiation. In active tracking, the markers themselves may be infrared LED's that emit infrared radiation that can be directly seen by the camera. Three or more markers may be arranged in a predefined geometry to give total specification of a unique vector with 6 degrees of freedom (DOF), three degrees of freedom in translation and three degrees of freedom in rotation. By altering the predefined geometry of the markers, the system can recognize and simultaneously track various probes and tools, including the special “reference probe” that defines the arbitrary origin in the physical space. Optical systems may also include software that performs image registration and navigational information to the end user.
Other example image guidance systems may employ magnetic field generators to generate a uniform gradient field to track spatial localizations. In these systems, a magnetic sensor may be used to measure the strength and direction of the magnetic field, and based on this information, spatial localization may be derived. Similarly, in these systems a reference point may be fixed to the patient and/or various probes may also be available for flexible navigation.
Another example image guidance system may be a stereotactic system. For cranial procedures, these systems may rely upon the attachment of a rigid frame around a patient's head. Cross-sectional imaging (e.g., CT, MRI, etc.) may be taken of the patient's head with the frame rigidly attached to patient's head. The frame may provide measurement of the physical space around the patient's head that directly correlates with the image space since the frame is simultaneously captured on the cross-sectional imaging scan. Thus, registration of the image space and physical space occurs automatically once a common arbitrary coordinate system is chosen on the scan.
Currently, guidance of surgical tools in these systems may be achieved mechanically (e.g., an external mechanism may direct the surgeon's instrument down a machined groove or bore). However, the surgeon must rely solely on trajectory calculations since no visual feedback is available in the absence of real-time imaging (e.g., intra-operative CT scanning, MRI scanning, etc.). Mechanical guidance can be expressed in various coordinate systems—Cartesian, polar, spherical, or mixed. Mechanical guides may rely on the “arc” principle, whereby the arc is always centered over the target. This may allow the surgeon to pick any ring or arc angle to find the most optimal placement of an entry site. Alternatively, an entry site may be predefined and arc/ring angles may be calculated. Various size guides may be available to accommodate various instrument diameters. However, since current systems cannot provide live image guidance, their roles may be limited to simple procedures, such as biopsies, placement of electrodes, etc.
Image navigation has proven to be extremely useful in improving accuracy of targeting, avoiding damage to surrounding critical structures, and improving patient outcomes. However, accurate targeting of deep anatomical structures is challenging across multiple disciplines. There is a need for an image guidance systems that facilitate identification of ideal trajectories that are difficult to visualize.
There are several clinical applications that may benefit from such improved targeting methods. One example is the insertion of external ventricular drains (EVD) or ventricular shunts (ventricular peritoneal, ventricular atrial, ventricular pleural, etc.). EVD procedures may be performed to release/redirect cerebrospinal fluid (CSF) and/or monitor intracranial pressure (ICP). The current standard of care in EVD procedures involves a blind passage of the ventricular catheter from the skin surface to the deep ventricular system in the brain via crude external landmarks. Current image guided systems used in this procedure rely upon rigid fixation of the head and access to the operating room. In addition, the use of existing image guided systems may significantly lengthen the procedure time, making their use in the emergency setting unsuitable, especially when urgent control of ICP is needed.
Another clinical application that may benefit from improved targeting methods is the performance of biopsies and related procedures. Accurate targeting of soft tissue, bone, fluid, or anatomical spaces may be used to facilitate biopsy, device placement, and/or pharmacological agent delivery. For example, a common cranial application is a stereotactic biopsy. Traditional methods have focused on frame-based stereotactic biopsy that relies upon the application of a frame secured to the skull with sharp pins that penetrate the outer table of the skull. This procedure is painful for the patient and cumbersome to set up. Recent advancements in image guidance systems have allowed the development of “frameless stereotaxy.” In this instance, the pre-procedural application of a frame followed by imaging of the patient with his/her head in the frame may be avoided. However, the head still needs to be rigidly fixed with penetrating pins in a skull clamp. With these systems, patients are typically given a general anesthetic because of the pain associated with fixating the skull and the immobilization that the patient experiences. Furthermore, in frameless stereotaxy systems the targeting information is shifted entirely to the guidance system and the screen requiring the surgeon to periodically look away from his or her hands and surgical instruments to view the screen for trajectory guidance.
Similar systems have been deployed to place electrodes or other implants. For instance, deep brain stimulator or RF ablation electrode insertion into cranial structures employs similar steps as a stereotactic biopsy. In this instance, the goal is to place an implant into a pre-defined area of the brain. Again, utilizing similar image-guided techniques, abnormal fluid or soft tissue collections including, but not limited to intracerebral abscesses, hematomas, or protein collections can be targeted.
There are numerous potential applications of the image-guided techniques disclosed herein for orthopedic procedures, ranging from placement of implants to placement of nails, plates, screws, and the like. For example, in hip replacement surgeries, accurate placement of the acetabular cap with specific angles of abduction/adduction and flexion/extension has been shown to be an important factor in preventing premature wear and recurrent hip dislocations. Similarly, knee, shoulder, ankle and small joint replacements rely upon precise cuts in the adjacent bones to ensure anatomical alignment of the implant. Another example includes the placement of pedicle screws in spinal surgery, which rely upon a precise trajectory and angle of insertion to prevent neurological injury and screw misplacement. An additional frequent orthopedic application involves the placement of intramedullary nails in long bone fractures. Intramedullary nails may conform to the shape of the intramedullary canal, sometimes making accurate targeting and alignment of distal locking screw holes difficult. Unfortunately, although many attempts have been made, no satisfactory system currently exists that can easily address this problem without significantly lengthening the operative time.
Unfortunately, all of these image-guided surgical techniques currently involve access to an image guidance system, a fixation method, and an operating room. Access to such facilities and instruments may not be feasible for emergency procedures, where the delay in bringing the patient to the operating room and setting up existing image guidance systems would result in a catastrophic outcome for the patient. In these instances, the physician is often forced to resort to crude external anatomical landmarks for rough guidance. This trade-off between speed and accuracy means that patients who require emergency procedures are often not able to receive the benefits of precise image-guidance. Further, existing image guidance systems are, in many instances, expensive and cost-prohibitive for smaller medical facilities. This means that image guidance technology is typically restricted to large, well-funded hospitals. Thus, many hospitals and healthcare facilities are not equipped with traditional image guidance systems, depriving patients of the benefits of the accuracy and precision of image-guided procedures. This is particularly true in developing countries where cost is a major barrier to the adoption of image guidance technology.
Additionally, routine radiology procedures such as biopsies are performed under the guidance of plain films, CT scans, ultrasound imaging, and magnetic resonance imaging. These procedures are performed frequently and may expose radiologists and technicians to harmful doses of radiation over time. Furthermore, all of these imaging modalities require practitioners to view an image on a screen, computer terminal, or the like, instead of watching the procedure in the physical space of the patient. Thus, when using existing image guidance systems, practitioners must take their eyes off the patient and focus on the information displayed on the screen (i.e., “eyes off target”). For these critical moments, the practitioners do not have direct visual confirmation of their instrument(s). Instead they must rely on feel, muscle memory, and/or rapidly looking back and forth between the screen and the patient. Therefore, a need exists for an image guidance system that can use previous imaging studies to guide the physician as they target a structure hidden below the surface of the skin, without the use of frames or pins, while providing direct visualization within the working area of the targeting trajectory to help practitioners keep their “eyes on the target” as they visualize/target structures inside the patient.